Yesterday the U.S. Climate Change Science Program released an assessment report titled "Weather and Climate Extremes in a Changing Climate" (PDF) with a focus on the United States. This post discusses some interesting aspects of this report, with an emphasis on what it does not show and does not say. It does not show a clear picture of ever increasing extreme events in the United States. And it does not clearly say why damage has been steadily increasing.

First, let me emphasize that the focus of the report is on changes in extremes in the United States, and not on climate changes more generally. Second, my comments below refer to the report's discussion of observed trends. I do not discuss predictions of the future, which the report also covers. Third, the report relies a great deal on research that I have been involved in and obviously know quite well. Finally, let me emphasize that anthropogenic climate change is real, and deserving of significant attention to both adaptation and mitigation.

The report contains several remarkable conclusions, that somehow did not seem to make it into the official press release.

1. Over the long-term U.S. hurricane landfalls have been declining.

Yes, you read that correctly. From the appendix (p. 132, emphases added):
The final example is a time series of U.S. landfalling hurricanes for 1851-2006 . . . A linear trend was fitted to the full series and also for the following subseries: 1861-2006, 1871-2006, and so on up to 1921-2006. As in preceding examples, the model fitted was ARMA (p,q) with linear trend, with p and q identified by AIC.

For 1871-2006, the optimal model was AR(4), for which the slope was -.00229, standard error .00089, significant at p=.01. For 1881-2006, the optimal model was AR(4), for which the slope was -.00212, standard error .00100, significant at p=.03. For all other cases, the estimated trend was negative, but not statistically significant.
2. Nationwide there have been no long-term increases in drought.

Yes, you read that correctly. From p. 5:
Averaged over the continental U.S. and southern Canada the most severe droughts occurred in the 1930s and there is no indication of an overall trend in the observational record . . .
3. Despite increases in some measures of precipitation (pp. 46-50, pp. 130-131), there have not been corresponding increases in peak streamflows (high flows above 90th percentile).
From p. 53 (emphasis added):
Lins and Slack (1999, 2005) reported no significant changes in high flow above the 90th percentile. On the other hand, Groisman et al. (2001) showed that for the same gauges, period, and territory, there were statistically significant regional average increases in the uppermost fractions of total streamflow. However, these trends became statistically insignificant after Groisman et al. (2004) updated the analysis to include the years 2000 through 2003, all of which happened to be dry years over most of the eastern United States.
4. There have been no observed changes in the occurrence of tornadoes or thunderstorms

From p. 77:
There is no evidence for a change in the severity of tornadoes and severe thunderstorms, and the large changes in the overall number of reports make it impossible to detect if meteorological changes have occurred.
5. There have been no long-term increases in strong East Coast winter storms (ECWS), called Nor'easters.

From p. 68:
They found a general tendency toward weaker systems over the past few decades, based on a marginally significant (at the p=0.1 level) increase in average storm minimum pressure (not shown). However, their analysis found no statistically significant trends in ECWS frequency for all nor'easters identified in their analysis, specifically for those storms that occurred over the northern portion of the domain (>35°N), or those that traversed full coast (Figure 2.22b, c) during the 46-year period of record used in this study.
6. There are no long-term trends in either heat waves or cold spells, though there are trends within shorter time periods in the overall record.

From p. 39:
Analysis of multi-day very extreme heat and cold episodes in the United States were updated from Kunkel et al. (1999a) for the period 1895-2005. The most notable feature of the pattern of the annual number of extreme heat waves (Figure 2.3a) through time is the high frequency in the 1930s compared to the rest of the years in the 1895-2005 period. This was followed by a decrease to a minimum in the 1960s and 1970s and then an increasing trend since then. There is no trend over the entire period, but a highly statistically significant upward trend since 1960. . . Cold waves show a decline in the first half of the 20th century, then a large spike of events during the mid-1980s, then a decline. The last 10 years have seen a lower number of severe cold waves in the United States than in any other 10-year period since record-keeping began in 1895 . . .
From the excerpts above it should be obvious that there is not a pattern of unprecedented weather extremes in recent years or a long-term secular trend in extreme storms or streamflow. Yet the report shows data in at least three places showing that the damage associated with weather extremes has increased dramatically over the long-term. Here is what the report says on p. 12:
. . . the costs of weather-related disasters in the U.S. have been increasing since 1960, as shown in Figure 1.2. For the world as a whole, "weather-related [insured] losses in recent years have been trending upward much faster than population, inflation, or insurance penetration, and faster than non-weather-related events" (Mills, 2005a). Numerous studies indicate that both the climate and the socioeconomic vulnerability to weather and climate extremes are changing (Brooks and Doswell, 2001; Pielke et al., 2008; Downton et al., 2005), although these factors' relative contributions to observed increases in disaster costs are subject to debate.
What debate? The report offers not a single reference to justify that there is a debate on this subject. In fact, a major international conference that I helped organize along with Peter Hoeppe of Munich Re came to a consensus position among experts as varied as Indur Goklany and Paul Epstein. Further, I have seen no studies that counter the research I have been involved in on trends in hurricane and flood damage in relation to climate and societal change. Not one. That probably explains the lack of citations.

They reference Mills 2005a, but fail to acknowledge my comment published in Science on Mills 2005a (found here in PDF) and yet are able to fit in a reference to Mills 2005b, titled "Response to Pielke" (responding to my comment). How selective. I critiqued Mills 2005a on this blog when it came out, writing some strong things: "shoddy science, bad peer review and a failure of the science community to demand high standards is not the best recipe for helping science to contribute effectively to policy."

The CCSP report continues:
For example, it is not easy to quantify the extent to which increases in coastal building damage is due to increasing wealth and population growth in vulnerable locations versus an increase in storm intensity. Some authors (e.g., Pielke et al., 2008) divide damage costs by a wealth factor in order to "normalize" the damage costs. However, other factors such as changes in building codes, emergency response, warning systems, etc. also need to be taken into account.
This is an odd editorial evaluation and dismissal of our work (Based on what? Again not a single citation to literature.) In fact, the study that I was lead author on that is referenced (PDF) shows quantitatively that our normalized damage record matches up with the trend in landfall behavior of storms, providing clear evidence that we have indeed appropriately adjusted for the effects of societal change in the historical record of damages.

The CCSP report then offers this interesting claim, again with the apparent intention of dismissing our work:
At this time, there is no universally accepted approach to normalizing damage costs (Guha-Sapir et al., 2004).
The reference used to support this claim can be found here in PDF. Perhaps surprisingly, given how it is used, Guha-Sapir et al. contains absolutely no discussion of normalization methodologies, but instead, a general discussion of damage estimation. It is therefore improperly cited in support of this claim. However, Guha-Sapir et al. 2004 does say the following on p. 53:
Are natural hazards increasing? Probably not significantly. But the number of people vulnerable and affected by disasters is definitely on the increase.
Sound familiar?

In closing, the CCSP report is notable because of what it does not show and what it does not say. It does not show a clear picture of ever increasing extreme events in the United States. And it does not clearly say why damage has been steadily increasing.

Overall, this is not a good showing by the CCSP.